Fiber optic communication involves modulating optical signals at high bit rates and transmitting the modulated optical signals over optical fibers. For example, in a wavelength division multiplexed (WDM) fiber optic communications system, optical carrier signals at a sequence of distinct wavelengths are separately modulated by information channels and then multiplexed onto a single optical fiber. Efforts continue toward increasing the data capacity of fiber optic communications systems, as well toward increasing the practical transmission distance of fiber optic spans. Although the development of erbium-doped fiber amplifiers (EDFAs) has substantially eliminated optical fiber attenuation as an obstacle to achieving longer transmission distances, other adverse effects suffered by optical fibers continue to serve as limitations on achievable bandwidths and/or distances.
Chromatic dispersion, termed dispersion herein, is one effect that limits the performance of an optical fiber span. Dispersion refers to a loss of signal shape as different component wavelengths travel down the optical fiber at different speeds. In practice, dispersion effects limit the rate at which a light beam at a given wavelength may be modulated with information (thereby limiting system throughput), and limit the allowable spacing between regenerators in a fiber optic communications link (thereby increasing system cost). For one widely used type of single mode fiber, the dispersion is about 17 ps/nm/km at a typical WDM wavelength near 1500 nm, wherein light at a wavelength of 1500 nm travels down the fiber 17 ps/km faster than light at a wavelength of 1501 nm, and travels down the fiber 17 ps/km slower than light at a wavelength of 1499 nm. Because the dispersion value for an optical fiber is usually dependent on signal wavelength, the dispersion characteristic for any particular type of optical fiber is often expressed as a plot of dispersion versus wavelength.
Dispersion compensating fibers (DCFs) are often used to compensate for the dispersive effects of optical fibers. Because of a core diameter much smaller than that of an ordinary optical fiber, the dispersion characteristic of a typical DCF is highly negative. When placed after (or before) the optical fiber span, the highly negative dispersion of the DCF compensates (or pre-compensates) for the positive dispersion of the optical fiber, thereby restoring the signal shape. Issues that arise for many DCFs include limitations on the amount of dispersion compensation per unit length, the high attenuation caused by the narrow core, and tunability limitations.
Chirped fiber Bragg gratings (chirped FBGs) can also be used for dispersion compensation. Chirped FBGs comprise segments of optical fiber into which lengthwise periodic variations of refractive index are “written” or burned, the periodic variations being chirped between longer periods at a first end and shorter periods at the opposite end. In operation, the chirped FBG receives the optical signal at the first end, and then reflects the wavelengths of interest back out that same first end, the shorter wavelengths being delayed relative to the longer wavelengths, thereby achieving a highly negative dispersion to compensate for the positive dispersion of the fiber span. Issues that arise for many chirped FBGs include the lengths of FBG fiber needed to provide dispersion compensation over a sufficiently wide spectral bandwidth, difficulty of fabrication, sensitivity to thermal variations, tunability limitations, and group delay ripple that can be a source of system noise.
It would be desirable to provide dispersion compensation in a manner that avoids, addresses, or improves upon one of more of the above-described issues associated with DCFs and chirped FBGs. Other issues arise as would be apparent to one skilled in the art upon reading the present disclosure.